Don't use guetzli on a transparent GIF
Don't use guetzli on a transparent GIF

A GIF file always has mode P.  Use "transparency" in img.info to detect
GIF files that include transparent in their palettes.  It can erroneously
think a file is transparent when it isn't, but that's OK.

See https://stackoverflow.com/questions/10689512/determining-if-a-gif-is-transparent-in-python

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